Okay, this is a weird one, but I'll try and explain as best as possible. We have two Exchange 2013 servers, we'll call them MAIL01 and MAIL02. The servers are members in a DAG. Normally, all mail databases are on MAIL01. This is the issue I'm having: when someone sends email from their mailbox, looking at headers shows that it goes from MAIL01 to MAIL02, then out to the smarthost (and final delivery). On receiving email, the mail comes into the spam filter (a Barracuda), goes to MAIL01, then to MAIL02, then back to MAIL01 for delivery (again, all mailboxes are on MAIL01).
Now, as a test, I moved a mailbox database to MAIL02 and had an email account from that database send and email. The headers indicate the email went from MAIL02, to MAIL02 again, then out to the smarthost. Incoming was similar: mail came in to the spam filter, then went to MAIL02, moved to MAIL02 again, then was delivered.
There seems to be something set somewhere that indicates MAIL02 is the master mail sender, and it is the only one allowed to move mail. I've spent hours pouring over settings and verifying everything I can. I found that the receive connectors are identical to eachother, the send connector looks correct, the DAG is functional (I even tested a failover), and both servers have Mailbox and Client Access roles.
Is this anything someone has encountered, and most importantly, knows how to fix?